This invention relates to microfluidics, to on-chip manipulation of analytes in fluid streams, and to monitoring for the presence and concentration of analytes in fluid (i.e., liquid or gas) streams.
For low levels of detection, bioassays usually require labels or tags which connect the recognition process to a transduction mechanism. Such labels include radioactive compounds, enzymes and light emitting materials. Radioactive labels are useful in that they offer a low level of detection, but they require expensive detectors, present a potential radiohazard, and can be unstable. In comparison, enzymatic labels are more stable, less hazardous, and less expensive, but are not as sensitive. Light emitting labels, though sensitive, require specialized detectors and labels, which are bulky and/or expensive.
In recent years there has been an increasing interest in magnetic labels for chemical and bioanalysis, as exemplified by the interest in immunomagnetic separation technology, which is a proven method for such tasks as monitoring parasites in raw surface water. In that particular example, the requirements of parasite filtration, concentration, separation, and monitoring require bulky instrumentation and manual operation. One such example is described in Kriz; C. B.; Radevik, K; Kriz, D. xe2x80x9cMagnetic Permeability Measurements in Bioanalysis and Biosensors,xe2x80x9d Anal. Chem. 1996, 68, 1966, in which a ferromagnetic sample is placed in a container which in turn is placed in a measuring inductor electrically connected in a bridge sensing circuit.
There have been investigations that have demonstrated the feasibility of magnetic detection concepts as applied to biomolecules. Insofar as applicants are aware, that work has been limited to attempts to utilize the high sensitivity of giant magnetoresistive sensors (GMRs) for antibody and DNA detection through immobilization of capture molecules on GMR surfaces. These investigations recognize the high sensitivity of giant magnetoresistive sensors for antibody and DNA detection, but require the mechanism of immobilization of capture molecules on GMR surfaces. The capture molecules bind the magnetically labeled target analyte resulting in a change in the GMR resistance. This approach, which requires recognition specificity of binding between the capture and target molecules, can suffer from non-specific interactions that will affect the GMR reading and consequently compromise the level of detection. It also requires the selective binding of the analyte directly onto the surface of the GMR. U.S. Pat. No. 5,981,297 discloses the binding to a giant magnetoresistive sensor of binding molecules which are capable of immobilizing the target molecules. The output of the devices is then a measure of the number of analyte or target molecules bound to the cells of the sensor. In this arrangement, the goal is for rapid detection stated to be on the order of 15-30 minutes per assay.
In view of the foregoing, it is the general aim of the present invention to utilize magnetic detection for monitoring analytes flowing in liquid and air streams.
In that regard it is an object to utilize the high sensitivity of the giant magnetoresistive sensor, and to position such a sensor in close proximity to a flowing stream containing the analyte.
It is a further object to utilize detection of such a sensor based on either the intrinsic magnetism of the analyte or on the magnetism of a label bound to the analyte.
In accordance with the invention there is provided a method of monitoring analyte flowing in fluid streams. A giant magnetoresistive sensor has a plurality of sensing elements that produce electrical output signals; the signals vary dependent on changes in the magnetic field proximate the elements. A stream including the analyte is provided, the stream having a magnetic property that is dependent on the concentration and distribution of analyte therein. The magnetic property can be imparted by use of ferromagnetic particles or by use of paramagnetic or superparamagnetic particles in conjunction with application of a magnetic field. The stream is flowed past the giant magnetoresistive sensor in sufficiently close proximity to cause the magnetic properties of the stream to produce electrical output signals from the GMR. Electrical signals are monitored as an indicator of the analyte concentration or distribution in the stream flowing past the GMR.
Apparatus for practicing the method includes a giant magnetoresistive sensor having a plurality of sensing elements for detecting localized changes in the magnetic field proximate the elements. Microfluidic channels are associated with the GMR sensor closely proximate the elements of the sensor. The proximity is such that the paramagnetic particles flowing in the channels will cause an output from the GMR sensor that is indicative of the concentration or distribution of magnetic particles. A source of analyte in a fluid stream is altered such that the fluid stream has a magnetic property that is related to the concentration or distribution of the analyte in the stream. The fluid source is connected to the microfluidic channels for flowing a stream including the analyte past the GMR sensor. An electrical monitor is responsive to the GMR sensor for measuring and recording changes in the output signal as an indication of the magnetic properties and therefore analyte concentration or distribution in the stream flowing past the GMR sensor.
Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.